In one widely used conventional spark plug, a noble metal tip protruding toward a center electrode is disposed on a ground electrode. For example, in a spark plug described in Japanese Patent Application Laid-Open (kokai) No. 2015-125879, a cuboidal noble metal tip is disposed on a ground electrode. In a spark plug described in Japanese Patent Application Laid-Open (kokai) No. 2008-270188, an elliptic cylindrical noble metal tip is disposed on a ground electrode. A noble metal tip having any of these shapes can easily have a larger volume than a cylindrical (circular cylindrical) noble metal tip, and the spark plug can therefore have improved durability.
In the spark plug in Japanese Patent Application Laid-Open (kokai) No. 2015-125879, the noble metal tip has a cuboidal shape, and the spark plug can therefore have improved durability. However, since the surface area of this noble metal tip is larger than that of a cylindrical noble metal tip with the same volume as the above noble metal tip, its quenching action is likely to be stronger, and this may lower the ignition performance of the spark plug. In the spark plug disclosed in Japanese Patent Application Laid-Open (kokai) No. 2008-270188, the noble metal tip has an elliptic cylindrical shape, and the spark plug can therefore have improved durability. However, since the minor axis direction of the noble metal tip that is a direction in which a flame core can easily expand extends along the lengthwise direction of the ground electrode, the expansion of the flame core is likely to be impeded by a base end portion of the ground electrode, and this may lower the ignition performance of the spark plug. Therefore, in a spark plug in which a noble metal tip protruding toward a center electrode is disposed on a ground electrode, there is a need for a technique that improves the ignition performance of the spark plug while ensuring the durability of the spark plug.
The present invention has been made to address the above problem and can be embodied in the following modes.